The Greatest Story Ever Told — 10 — The lights are on, can you see?

Perhaps in time the so-called Dark Ages will be thought of as including our own.
Georg Lichtenberg

We’ve been going through the history of the Universe — from inflation to the present day — and you can read parts 1, 2, 3, 4, 5, 6, 7, 8, and 9 here. The last thing that happened was we had a Universe filled with neutral atoms — mostly hydrogen with some helium and a negligible amount of everything else — that had begun to collapse under its own gravity.

When a few regions of space got dense enough, about 50 million years into the story, we made the first stars!

But there’s a problem when you make stars surrounded by neutral atoms that you may not be aware of: neutral atoms block light! If we look around in our own galaxy, it’s pretty easy to find where neutral atoms are, because the light doesn’t get through!

Astronomers call this phenomenon extinction. So, what does this mean for these first stars? It means that, as long as this neutral gas is around, the Universe is opaque to starlight, and we can’t see it! At least, not until this gas gets ionized. What ionizes gas?

These very same hot, young stars that form from gravitational collapse! Most of a Sun-like star’s energy comes out in visible light, but a hotter, more massive star emits more Ultraviolet (UV) light. What’s special about UV light? It smacks these neutral atoms and knocks electrons out, ionizing them! This means that, once enough stars form, more and more of the Universe becomes transparent to visible light!

And finally, when the Universe has had enough stars on for long enough, these so-called Dark Ages end, and you can actually observe the light!

So, again, this is a prediction of the Big Bang. Sometime, if we look back far enough, these atoms that are ionized in between galaxies today should be neutral, and should absorb the light! So what happens when we confront this prediction (made in 1985, by Jim Gunn) with the observations?

We find that once things are far enough away (and hence, young enough), we can’t see any light, which is why there are no features to the left of the more distant (higher redshift) object! In fact, if we take a slew of these distant objects, you’ll see very clearly that the highest redshift ones (most distant) are totally muted to the left of the big hydrogen emission line!

So making the first stars isn’t enough, we need to give them time to work their nuclear fusion engines to ionize all the gas in the Universe in order to see! And this wonderful prediction was only confirmed in 2001, and has since been confirmed by many subsequent observations, including measurements of ionization by the WMAP satellite!

So this brings us all the way up to the Universe being almost one billion years old! Come back next time, and we’ll find out what else has been going on, and for some gorgeous early pictures of our Universe!